1. Field of the Invention
The present invention relates to a communication system, and more particularly, to a method and apparatus for scanning wireless channels.
2. Description of the Related Art
Wireless local area network (WLAN) technology is now widely used in various applications. Numerous organizations devote extensive resources to research seeking improvements in WLAN data communication quality. In a WLAN, a wireless transmit/receive unit (WTRU) may be located within communication range of several access points (APs). However, the AP associated with the WTRU and the channel the WTRU operates on may change over time due to signal strength variations. On the other hand, when is the WTRU roams among various APs, the channel utilized may be adjusted in accordance with a location of the WTRU and the signal quality. Before the WTRU can be associated with an AP, a handoff procedure has to be performed. The handoff procedure includes a scanning phase, an authentication phase and a re-association phase, and the scanning phase incurs significant delay.
FIG. 1 illustrates a diagram for a wireless network. The network comprises a station 11, and AP 12, 13. The operating channel of the AP 12 is channel 1, and the operating channel of the AP 13 is channel 2. A channel scanning procedure has to be performed for the station 11 to obtain the related information of all APs within the signal communication range of the station 11. The related information includes service set identifiers (SSIDs), the signal strength of every detected AP, encryption modes, etc. The two currently available modes for scanning are active scanning and passive scanning. However, both active scanning and passive scanning interrupt the operations to transmit or receive packets. If the station 11 is a dual-band station that can be operated on 38 operating channels, and 140 ms is required to scan each channel, then the station 11 needs 5320 ms to scan all channels. That is, the station 11 would need to stop communicating with an associated AP for 5320 ms in order to complete the operation of scanning all channels. Because the data forwarding and data receiving operations are interrupted during this period, important packets could be lost. Therefore, finding ways to reduce the scanning time and the data loss rate during the scanning operation is an important issue for the market.